Well, not to further complicate things for you, but I hear the snorkeling off the shore is better on the west coast (ie. Perth area). I've never been so I can't speak from experience, but from what I'm told, the west coast is not mountainous so there's no rain run off like there is on the east coast (which makes the water brownish). (Unfortunately this is where places like Samoa, Fiji, Tahiti, Vanuatu, etc. have the "edge" - those places you CAN just snorkel off the beach and see South Pacific reef. It's just that generally speaking you pay dearly for that extra.) But the west coast is definitely a hard thing to fit in, I would think, it's like spending 3 weeks in Canada and planning on seeing Halifax AND Vancouver.
Anyhow, back to the GBR.
I'm not a diver (no certification - I want to get it "some day" but I don't yet have it), but the dive companies can still take you out to areas where you can snorkel. The place I used (I used them twice), one was out the "Low Isles" (sadly that's basically the spot where Steve Irwin met his match), and the other was just to some random reef spots. Both trips you could either dive, or snorkel. In fact the outer reefs trip took us to 3 different locations, and you could either dive for real (if you had your certification), dive with an instructor (even if you've never dived before - they give you a cursory lesson and you're in a group of no more than 4 people to an individual instructor and you don't go deeper than about 15 feet), or you could snorkel. I ended up trying the dive thing once, then snorkeled at the next, and then decided I would dive again on the third outing. The water was 26 degrees, to a Canadian this might have been a nice warm bath. But they give you wetsuits if you want (to the Aussies it was cold).
Anyhow the lack of depth on the rudimentary dive isn't really an issue because most of the most colourful stuff is up there right at the surface anyhow.
A liveaboard dive trip simply means they take you out there on the boat ... and you stay out there for a couple of days. Meals and sleeping is all on the boat. It'd probably be a wicked cool adventure and I have some friends who just did a few days sail trip around the Whitsundays (oh. Right. The Whitsundays. Add that too to the list of "places I have to go see"). They weren't divers either (no certifications) but were able to do some diving on the GBR on this trip.
So generally speaking, even if don't dive, it's not a showstopper, it just means you can't go out and do some of the more advanced things offered.
I think the company I used was "Quicksilver". I don't know if they still exist, I was there in 2001 and I guess that was a long time ago now.
.. Oh, Ok, I see they still operate. Here's their website. I see that the two trips I did with them are still offered: Low Isles and Agincourt Reef. Here's their website:
http://www.quicksilver-cruises.com
(Oh man, looking at the pictures on their website, I sooooo want to go again.
Sigh, set up a new tank, or go traveling???
What to do, what to do..)
... Anyhow, we did rent a car for the time we were in Queensland. When I was in NSW, I just kinda stole one of my sisters family's cars since we were crashing at their place anyhow.
Right-hand driving isn't too bad, it doesn't take that long to get used to, the pedals are the same and if you have a stick shift, it's still mapped out the same and everything. The biggest thing I found was that some brands of cars switch the turn signals and window wiper levers (but not all makers, so it's inconsistent). I found I was *constantly* turning on the wipers everytime I went to do a turn... Oh, and roundabouts. They're pretty big on roundabouts down there (much like any British-ish place, I guess). The only real scary moment I ever had was when I was on a bus coming back from Port Douglas back down to Cairns (the Quicksilver company sends a bus to pick you from where you're staying), there was a moment where we rounded a bend on the highway and there was a a car on the wrong side of the highway coming up towards us. The driver slammed on the brakes and the bus fishtailed a bit, but other than a lot of people needing new shorts, an incident was averted.
Anyhow I found a trick to help you remember what side to be on, is that, if you're driving, you're on the side of the car that should be nearest the centre of the road.
Cairns also has other things to do, there's Tjapukai (an Aborginal cultural centre), there's a steam train and a gondola that goes up into the mountains (the tropical rainforest there is really worth seeing, it's like everything you ever imagined what a jungle looks like.. except it's a real jungle, not your imagination). Check it out, they offer package deals to do the train, gondola, and Tjapukai as a day trip:
http://www.skyrail.com.au/
Sorry for the long winded reply. I guess I could go on and on about this place. As far as cities go, Cairns may as well be Okotoks or Airdrie, or Brooks, or maybe Red Deer. (I'm not knocking those cities, but I guess what I'm saying it's not a sophisticated large urban centre, it's very much a small town. But it just happens to be in the tropics and near to perhaps the most awesome reef there ever was, sooooooo.... really, how can you go wrong??
)